Bird Networks in the Amazon (complexity)

Relationships between organisms are broad

  • Competition
    • today


  • Mutualisms
    • jigsaw


  • Predation and Consumption
    • next week


  • Parasites and hosts


  • Humans vs other species
    • data exercise

What is competition?

WHO: Intra vs Inter-specific competition

Competition always reduces the fitness of both parties

Intraspecific competition limits population growth

Competition is a powerful driver of evolution




  • Natural selection favors individuals that do not compete


  • Drives niche differentiation, trait specialization and potentially species diversification


  • Natural selection can cause character displacement in areas where overlap is high

Gause’s Law: Theory of Competitive Exclusion





  • Two species cannot occupy the exact same niche if they need the same resource


  • Those species cannot coexist at stable constant populations
    • a winner and loser will emerge


  • Leads to local extinction of weaker competitor or shift to a different ecological niche

Gause’s test tube Paramecium study

Red vs Grey Squirrel in Britain (Murphy 2015)

Competitive exclusion in nature?

Competitive exclusion is hard to detect in nature



  • Resources are variable/limiting in an environment
    • leads to more symmetric competition (both struggle)


  • Species compete for multiple resources
    • exclusion based on a single resource is too simple


  • Complete niche overlap (test tube) is rare


  • TAKE HOME: Greater overlap = greater competition

Interspecific competition results in the ‘Realized Niche’

Joseph Connel: Competition among barnacles

Can’t we all just get along?

Evolution should also favor coexistence…

Co-existence and natural selection

  • Overlapping species may alter resource use to minimize competition
    • resource partitioning = division of resources


  • Species whose niches only partly overlap coexist easier
    • use the environment differently = niche differentiation
    • evolve traits to use different/less overlapping resources = character displacement


  • Co-existing species can overlap by using different forms of resources
    • reduces/eliminates competition

Co-existence through resource partitioning in lizards

Niche differentiation: different use of microhabitats

Here, different insects forage for resources at different locations and times

Character displacement in cicada mating calls